Inductively coupled plasma (ICP) sources have advantages over other types of plasma sources when used with a focusing column to form a focused beam of charged particles, i.e., ions or electrons. An inductively coupled plasma source, such as the one described in U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention, is capable of providing charged particles within a narrow energy range, thereby reducing chromatic aberrations and allowing the charged particles to be focused to a small spot.
ICP sources typically include a radio frequency (RF) antenna mounted coaxially around an insulating plasma chamber. The RF antenna provides energy to ignite and maintain a plasma within the plasma chamber. Typically, RF antennas comprise a coil with a multiplicity of coaxial turns displaced axially with respect to each other, wherein the overall length of the RF antenna controls the axial extent of the plasma generated within the plasma chamber. RF antennas may be longer due to increased numbers of turns or due to larger inter-turn spacings—in this case, the plasma will also be longer within the plasma chamber. RF antennas may be shorter due to decreased numbers of turns or due to smaller inter-turn spacings—in this case, the plasma will be shorter within the plasma chamber. Increased numbers of turns may improve the coupling efficiency between the RF power supply which energizes the RF antenna and the plasma within the plasma chamber. However, a shorter coil may produce a more efficient plasma for ion generation since, for a given input RF power, the plasma density may be roughly inversely proportional to the axial length of the plasma, assuming the plasma diameter within the plasma chamber remains constant.
Thus, the preferred antenna configuration is the largest number of turns in the antenna coil within the shortest possible length. Clearly, this implies that closer inter-turn spacings (i.e., the distances between successive turns in an RF antenna) should be as small as possible. However, the power from the RF power supply generates high voltages across the antenna coil, inducing voltages between successive turns which may exceed 400 Vrf and may go up to several kVrf at high RF powers. Increasing the RF power usually leads to increased plasma density and higher source emission currents; however, high RF power places more demanding high voltage standoff requirements on the antenna turn-to-turn insulation, as well as on the insulation between the antenna and neighboring electrical components at different potentials.
As discussed in the prior art, a Faraday Shield structure made of highly conductive metal may be placed between the plasma chamber and the antenna to minimize capacitive RF voltage coupling into the plasma, which can dramatically increase the source beam energy spread. Focused ion beam systems require the lowest possible energy spreads to reduce chromatic aberrations which would be detrimental achieving fine probe beams. Thus, the optimal antenna design must provide adequate voltage standoff between turns and between the antenna and its neighboring electrical components, such as a Faraday Shield.
What is needed then is an improved inductively coupled plasma ion source for use in a focused ion beam system.